Selective Oxidation of Cyclohexene with H2O2 Catalyzed by Resin Supported Peroxo Phosphotungstic Acid Under Mild Conditions

Article abstract of DOI:10.1007/s10562-020-03273-1

Abstract: A series of modified chloromethyl polystyrene resins loaded with peroxo phosphotungstic acid catalysts were synthesized for the selective oxidation of cyclohexene. The surface of resin was enriched with high concentration quaternary ammonium salt, and grafted with a large amount of peroxo PW-anion through ion exchange. The novel resin catalyst showed excellent cyclohexene conversion and epoxide selectivity using 30% H₂O₂ as oxidant at ambient temperature. Furthermore, the resin catalyst exhibited excellent recycling stability, which can be reused by a simple filtration and the peroxo phosphotungstic acid did not leach into the solvent after reaction. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03273-1

Catalysis Letters
https://doi.org/10.1007/s10562-020-03273-1
Selective Oxidation of Cyclohexene with H O Catalyzed by Resin
2
2
Supported Peroxo Phosphotungstic Acid Under Mild Conditions
Jia Liu1 · Guoqiang Yang · Ying Liu · Zheng Zhou · Zhibing Zhang · Xingbang Hu
1
1
1
1
1
Received: 14 January 2020 / Accepted: 21 May 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A series of modiꢀed chloromethyl polystyrene resins loaded with peroxo phosphotungstic acid catalysts were synthesized
for the selective oxidation of cyclohexene. The surface of resin was enriched with high concentration quaternary ammonium
salt, and grafted with a large amount of peroxo PW-anion through ion exchange. The novel resin catalyst showed excellent
cyclohexene conversion and epoxide selectivity using 30% H O as oxidant at ambient temperature. Furthermore, the resin
2
2
catalyst exhibited excellent recycling stability, which can be reused by a simple ꢀltration and the peroxo phosphotungstic
acid did not leach into the solvent after reaction.
Graphic Abstract
Keywords Resin · Oxidation · Cyclohexene · H O · Recyclable
2
2
1
Introduction
Cyclohexene oxide is one of the most essential chemi-
cal feedstocks for producing polymers, pharmaceuticals,
ꢀne chemicals and biological materials. The development
of eꢁective industrial production for the epoxidation of
cyclohexene has drawn much attention over the last few
years. The two main traditional methods for producing
epoxides are the hydroperoxides process and the chloro-
hydrin process. However, these processes produce a large
number of low-value by-products and cause severe environ-
mental pollution. As a replacement for traditional method,
environmentally benign oxidants, such as hydrogen perox-
ide [1–5] and molecular oxygen [6–11] have been used for
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03273-1) contains
supplementary material, which is available to authorized users.
*
Zhibing Zhang
zbzhang@nju.edu.cn
*
Xingbang Hu
huxb@nju.edu.cn
1
School of Chemistry and Chemical Engineering,
Separation Engineering Research Center, Key Laboratory
of Mesoscopic Chemistry of MOE, Nanjing University,
Nanjing 210093, People’s Republic of China
Vol.:(0
1
123456789)
3

J. Liu et al.
the epoxidation of alkenes. It has been reported that a high
selectivity of epoxides could be obtained from the epoxida-
tion of cyclohexene using hydrogen peroxide as an oxidant.
Many catalysts were developed for this process, including
heteropoly acid [12, 13], TS-1 [14–16], Schiꢁ base com-
plex [17], non-heme iron [18], and metal porphyrin [19].
Phosphotungstic heteropoly acid is a very active catalyst for
the epoxidation of alkene by aqueous H O [13, 20]. The
2 Experimental
2.1 Catalyst Preparation
The chloromethyl polystyrene resin (PS-Cl) used here is a
commercial product (Cross-linked with 1% DVB, Particle
size: 100–200 mesh, chloromethyl content: 3.0 mmol/g), the
cyclohexene (no stabilizer added) was purchased from TCI,
PEI (M.W. 600, branched) was purchased from Adamas. The
mixture of 10 g PS-Cl resin, 0.8 g PEI and 40 mL DMF
were stirred at 303 K for 4 h (Scheme 1). The resulting
PS/PEI was ꢀltered oꢁ and puriꢀed in a soxhlet extraction
(Et O–CH Cl (1:1)) to remove the excess PEI. Next, the PS/
2
2
activity of this epoxidation system is due to the formation
of metal peroxo species in situ, which are the real active
oxidants [13]. Most of the phosphotungstic acid catalysts
reported are liquid biphasic and homogeneous system, with
a quaternary ammonium cation playing the role of phase
transfer of peroxo phosphotungstic acid between aqueous
and organic layer [20–24]. However, this system suꢁers
from the problem of poor catalytic reactivity in the recy-
cling processes.
2
2
2
PEI support was dried at 353 K for 12 h. The dried support,
50 mmol 1-chlorooctadecane and 25 mL 2-propanol were
added into a 100 mL round bottom ꢂask (reꢂux, 353 k, 6 h).
Then, it was cooled down to 323 K. After that, 100 mmol
1-chlorobutane was added and the mixture was reꢂuxed at
From the view of industrial manufacture and green chem-
istry, heterogeneous catalysts are desirable because of their
separability, high stability, long lifetime and recyclability
323 K for 5 h. The resulting PS/PEI-C
was ꢀltered oꢁ
8
1
and puriꢀed in a soxhlet extraction, then dried at 353 k for
2 h. Peroxo phosphotungstic acid compounds H [PO (OH)]
[
25]. In 1999, Aida et al. reported a reusable resin-supported
1
2
3
catalyst (PW-Amberlite) [26]. Although high yields were
obtained with PW-Amberlite for the epoxidation of terpenes
[
WO(O ) ] (H PW ), H [PO (WO(O ) ) ] (H PW ) and
2 2 2 2 2 3 4 2 2 4 3 4
H (PhPO )[WO(O ) ] (H PhPW ) were prepared according
2
4
2 2 2
2
2
with H O , the catalytic performance for the selective oxi-
2
2
to the methods reported in the literatures [29–31]. The cata-
lysts with diꢁerent peroxo PW-anions were prepared through
ion exchange treatment of 5 g PS/PEI-C with excessive
dation of cyclic oleꢀns was weak [27]. In 2016, Peng et al.
reported a powdered anion-exchange resin D201 supported
peroxo PW-anion (D201-PWAR(4)), D201-PWAR(4) exhib-
ited high activity in the cyclohexene epoxidation using 30%
H O at 323 K, 92.4% cyclohexene conversion and 98.1%
1
8
aqueous solution of diꢁerent peroxo phosphotungstic acid
compounds (5 mmol PW-anion), and named as PS/PEI-
C -PW , PS/PEI-C -PW , PS/PEI-C -PhPW (Scheme
2
2
18
2
18
4
18
2
epoxy cyclohexane selectivity were obtained [28].
S1(A)). Finally, the solution was removed by ꢀltration, and
the resin catalysts were washed consecutively with deion-
ized water and acetone, then dried at 353 K for 6 h. Using
a similar method, the PS-C -PW was prepared through
In the present work, we developed a series of active cata-
lysts based on chloromethyl polystyrene resin (PS-Cl). By
surface modiꢀcation with polyethyleneimine (PEI), a large
amount of quaternary ammonium salt was grafted onto the
surface of resin. Through ion exchange of peroxo phospho-
tungstic acid with modiꢀed resin, an active heterogeneous
catalyst was obtained, which showed excellent catalytic per-
formance for the selective oxidation of cyclohexene with
1
8
2
ion exchange treatment of 5 g PS-C with 5 mmol H PW
1
8
2
2
(Scheme S1(B)).
2.2 Catalyst Characterization
3
0% H O under mild conditions (308 K). This catalyst
Fourier transform infrared (FT-IR) spectra of samples were
determined by Nicolet NEXUS 870 spectrometer using KBr
as background atmospheric conditions. Elemental contents
of C and N were measured by Elementar Vario ELII. Con-
tent of P and W elements were performed in a PE Optima
2
2
has a millimetre-level particle diameter, which is easy to
be separated and reused, showing potential for industrial
application.
5300DV elemental analyzer (ICP-AES). X-ray photoelectron
Scheme 1 Catalytic oxidation
of cyclohexene
1
3

Selective Oxidation of Cyclohexene with H O Catalyzed by Resin Supported Peroxo…
2
2
spectroscopy (XPS) was carried out by using a PHI 5000
VersaProbe spectrometer equipped with an Al Kα X-ray
source in ultrahigh vacuum (10–10 Torr). The SEM images
Table 1 Characteristics of support and catalysts
Support and ctalalysts
C%
N%
P%
W%
SBET; VBJH
PS/PEI
PS/PEI-C -PW
PS/PEI-C -PW
PS/PEI-C -PhPW
80.6
66.3
52.5
67.1
76.8
2.5
1.8
1.4
1.7
0.2
–
–
14.3
34.5
14.2
1.7
27.7; 0.27
17.8; 0.12
–
–
were observed by Hitachi S-3400 N. The N adsorption iso-
2
1.2
1.7
1.2
0.1
therm was measured by Micromeritics ASAP 2020 at 77 K.
The surface area was observed from the BET plot, and the
pore volume was performed by the BJH method. Thermo-
gravimetric analysis (TGA) was performed on Netzche
STA449F3 at a heating rate of 10 K/min from 298 to 1073 K
18
2
18
4
18
2
PS-C -PW
22.2; 0.22
18
2
Chemical composition of support and catalysts, weight%; speciꢀc
under N ꢂowing.
2
−1
3
−1
2
surface area, S , m g ; pore volume, V , cm g
BET BJH
2
.3 Catalyst Testing
can be enriched on the resin through surface modiꢀcation
by PEI, and more peroxo PW-anion can be grafted onto the
surface of the resin. As showing in Fig. S2, the TGA curves
of diꢁerent stages of the resin show high thermal stability
of resin at temperatures below 400 K. According to the IR
spectrum (Fig. S3), the PS-Cl is characterized by the follow-
The selective oxidation of cyclohexene was carried out in
a round bottom ꢂask (25 mL) equipped with a reꢂux con-
denser (263 K ethanediol as a cooling medium for the con-
densing to prevent the evaporation of cyclohexene and sol-
vent) (Scheme 1). Typically, 500 mg catalyst, 10 mmol of
cyclohexene, 20 mmol of H O (30 wt% solution in water),
−
1
ing IR bands (in cm ): 2920 (antisymmetric vibration of
C–H); 2850 (symmetric vibration of C–H); 1500 and 1600
(benzene skeleton vibration); 680 (stretching vibration of
C–Cl). In the spectrum of the PS/PEI, the absorption peak
2
2
5
mL of CH CN were added into the ꢂask. The mixture
3
was heated in an oil bath and stirred by a magneton. After
the reaction, the resin catalyst was ꢀltered oꢁ and washed
consecutively with deionized water and acetone, then dried
at 353 K overnight for reuse. The substrates and products
were analyzed by Shimadzu GC 2014C with a capillary
column WondaCAP-5 (30 m × 0.32 mm × 0.25 μm) and
a ꢂame ionization detector. The cyclohexene and prod-
ucts were determined by comparing the responsive peak
areas of sample products against standard curves prepared
with known authentic compounds using internal standard
decane. The conversion of cyclohexene and the selectivity
of epoxy cyclohexane were calculated by Eqs. (1) and (2),
respectively.
−1
ν(C–Cl) at 680 cm reduced signiꢀcantly, at the same time,
−1
the ν(C–N) peak appears at 1361 cm , which shows that the
C–Cl bond replaced by the C–N bond and the PEI had been
−
1
grafted onto the PS-Cl. New peaks (ν(P=O) at 1070 cm ,
−
1
−1
ν(W=O) at 940 cm , ν(W(O )) at 540 cm , and ν(O–O) at
2
−
1
820 cm ) are observed in the spectrum of PS/PEI-C -PW
1
8
2
[20, 32–34]. These results suggest that the PW anion had
2
been immobilized on the surface of resin. The IR spectra of
PS-C -PW , PS/PEI-C -PW and PS/PEI-C -PhPW are
1
8
2
18
4
18
2
similar to the PS/PEI-C -PW .
1
8
2
(
1)
The conversion of cyclohexene (mol%)
=
total moles of products∕(total moles of products + moles of remaining cyclohexene)
The selectivity of epoxy cyclohexane (%) = moles of epoxy cyclohexane∕total moles of products
(2)
3
Results and Discussion
The elemental compositions of resin surface were ana-
lyzed by XPS. Figure S4(A) shows the presence of Cl 2p
(BE = 200 eV), C 1s (BE = 285 eV), O 1s (BE = 531 eV),
this is consistent with the chemical composition of PS-Cl.
Figure S5(a) shows the Cl 2p core-level spectrum with Cl
2p located at 199.6 eV and Cl 2p located at 200.8 eV,
3.1 Catalyst Characterization
The SEM pictures of resin surface morphology in diꢁerent
stages are shown in Fig. S1. The shape and size of PS/PEI
are similar to PS-Cl (50–100 μm), the surface of PS/PEI
is rougher than that of PS-Cl. As shown in Table 1, after
grafted with PEI, the mass percentage of N in PS/PEI is
3
/2
1/2
which are characteristic peaks of C–Cl [35]. A new char-
acteristic peak of N 1s appears at 399.1 eV (Fig. S4(B))
[36]. The Cl 2p core-level spectrum of PS/PEI with Cl 2p
3
/2
2
0
.5%, the total pore volume and surface area of the resin is
at 197.3 eV and Cl 2p at 198.8 eV (Fig. S5(b)), which
1/2
3
−1
2
−1
.27 cm g and 27.7 m g . The mass percentage of N,
are characteristic peaks of free chloride ion [37]. The peak
+
P and W in PS/PEI-C -PW are much greater than that of
area of C–N of PS/PEI-C -PW is obviously increased
1
8
2
18
2
PS-C -PW . This means that more quaternary ammonium
compared to PS/PEI (Figs. S5(c), S5(d)), indicating that
1
8
2
1
3

J. Liu et al.
Table 2 Eꢁect of various catalysts on the epoxidation of cyclohexene
Catalysts
n(PW-anion) Conv
Sel.(%)
/(mmol) (mol%) epoxy- -diol others
PS/PEI-C18
0
0
–
–
1.5
10.1 2.0
5.3
0
−
0.3
PS/PEI-C -PW
0.200
0.282
73.2
84.2
74.2
9.1
98.2
87.9
91.3
100
18
2
PS/PEI-C -PW
18
4
PS/PEI-C -PhPW 0.202
3.4
0
18
2
PS-C -PW
2
0.018
18
Reaction conditions: catalyst (500 mg), CH CN (5 mL), cyclohexene
3
(
10 mmol), H O (20 mmol, 30 wt% solution in water), rpm (600),
2
2
3
08 K, 180 min
quaternization reaction has occurred on the surface of PS/
PEI. A strong W 4f peak (35.0 eV) and O 1s peak (530.1 eV)
appear, and the peak of Cl 2p is weakened (Fig. S4(C)),
because the peroxo PW-anion has replaced the chloride ion
Fig. 1 Cyclohexene epoxidation at normal operating conditions: con-
version of H O (a); utilization of H O (b); conversion of cyclohex-
2
2
2
2
ene (c); selectivity of epoxy cyclohexene (d). Reaction conditions:
(
on the surface of resin) by ion exchange. The W 4f core-
PS/PEI-C -PW (500 mg), CH CN (5 mL), H O (20 mmol, 30 wt.%
1
8
2
3
2
2
level spectrum of PS/PEI-C -PW with W 4f at 35.6 eV
1
8
2
7/2
solution in water), cyclohexene (10 mmol), rpm (600), 308 K
and W 4f at 37.6 eV (Fig. S5(e)), W 4f belongs to PW .
5
/2
7/2
2
Figure S5(f) shows the O 1s core-level spectrum, 530.1 eV is
ascribed to W–O band, 531.2 and 532.6 eV can be assigned
increase of solution polarity will promote the hydrolysis of
epoxy cyclohexene to 1,2-cyclohexanediol. The utilization
coeꢃcient of H O increased from 4.8 to 62.7%, with an
to the W–O–W and W–O–P bands in PW [28, 32, 38].
2
2
2
3.2 Catalyst Performance Evaluation
increase in reaction time. Hydrogen peroxide reacts with
phosphotungstic acid to form tungsten peroxo species, and
the tungsten peroxo species can catalyze the epoxidation
of cyclohexene as real active oxidants. The reaction rate of
forming tungsten peroxo species is much higher than the
speed of epoxidation of cyclohexene, so the utilization coef-
ꢀcient of H O was only 4.8% at the beginning of the reac-
As shown in Table 2, resin catalysts supported with various
phosphotungstic acids were investigated for catalytic activ-
ity. The cyclohexene conversion was 0% when PS/PEI-C
1
8
was used, indicating that the support has no catalytic activ-
ity. Only 9.1% cyclohexene conversion was obtained with
2
2
1
00% selectivity of epoxy cyclohexane in the presence of
tion (detection method of the concentrations of hydrogen
peroxide is mentioned in supporting information).
PS-C -PW (The amount of PW-anion is only 0.018 mmol).
18
2
In contrast, catalysts grafted with PEI presented an excellent
catalytic activity with more than 70% conversion, which was
remarkably higher than that of PS-C -PW . Because the
Furthermore, we designed the control experiment to con-
ꢀrm the stability of catalyst PS/PEI-C -PW . As shown in
1
8
2
Fig. 2, PS/PEI-C -PW was used to catalyze the cyclohex-
1
8
2
18
2
contents of PW-anion in catalysts modiꢀed by PEI are much
higher than that of the catalyst not modiꢀed by PEI. In addi-
tion, phosphotungstic acid catalysts with various structures
showed diꢁerent conversions and selectivities. Based on the
above results, PS/PEI-C was considered to be better cata-
ene oxidation at 308 K for 60 min (the ꢀrst run), then the cat-
alyst was ꢀltrated and was put into another glass ꢂask ꢀlled
with the same reactant, and the reaction was also carried
at 308 K for 60 min (the second run). The ꢀltered solution
after the ꢀrst run was transferred to an empty glass ꢂask and
reacted at 308 K for 60 min. The conversions of H O and
1
8
lyst support than PS-C , and PS/PEI-C -PW was selected
1
8
18
2
2
2
as the most suitable catalyst because of its highest epoxy
cyclohexene reached 36.8 and 24.6% respectively in 60 min
cyclohexane selectivity (98.2%).
for the ꢀrst run (Fig. 2a , b ). In contrast, the conversions of
1
1
Figure 1 shows the performance of PS/PEI-C -PW for
H O and cyclohexene of the reaction of the ꢀltered solution
1
8
2
2 2
the epoxidation of cyclohexene with H O . The conversion
were almost unchanged (Fig. 2c , d ). On the other hand,
1 1
2
2
of cyclohexene can arrive 95.9% in 360 min and the selectiv-
ity of epoxy cyclohexane was almost 100% in the ꢀrst two
hours. Then, the selectivity of epoxy cyclohexane decreased
to 92.1 and 65.5% in 180 min and 360 min, respectively.
The decrease in epoxy cyclohexane selectivity is due to the
enriched polar species in solution (e.g., water, epoxide), the
for the PS/PEI-C -PW catalyst that used for the second
18 2
time (Fig. 2a , b ), no reduction of the catalytic activity was
2
2
found compared to the ꢀrst time. The same results could
be obtained by repeating all the above operations (Fig. 2c ,
2
d , a , b ). These results indicated that the PS/PEI-C -PW
2
3
3
18
2
catalyst could be reused by a simple ꢀltration, and the peroxo
1
3

Selective Oxidation of Cyclohexene with H O Catalyzed by Resin Supported Peroxo…
2
2
Fig. 2 Stability of PS/PEI-
C -PW catalyst: conversion of
18
2
H O for catalyst ꢀrst/second/
2
2
third run (a /a /a ); conversion
1
2
3
of cyclohexene for catalyst ꢀrst/
second/third run (b /b /b ); con-
1
2
3
version of H O of the reaction
2
2
of the ꢀltered solution after the
ꢀ
rst/second run (c /c ); conver-
1 2
sion of cyclohexene of the reac-
tion of the ꢀltered solution after
the ꢀrst/second run (d /d ).
1
2
Reaction conditions: catalyst
(
500 mg), CH CN (5 mL), H O
3 2 2
(
20 mmol, 30 wt% solution in
water), cyclohexene (10 mmol),
rpm (600), 308 K
phosphotungstic acid did not leach into the solvent in this
reaction conditions.
98.0% in 180 min, as the reaction temperature rose from
298 to 308 K. As the reaction temperature rose to 313 K,
the cyclohexene conversion increased from 73.2 to 84.2%
and the epoxy cyclohexane selectivity sharply decreased
from 99.2 to 87.8% in 3 h, attributed to the hydrolysis of
epoxy cyclohexane to 1,2-cyclohexanediol at higher reac-
tion temperature.
The eꢁects of diꢁerent solvents on the cyclohexene epox-
idation have been studied (Table S1). A higher cyclohexene
conversion could be obtained in water-miscible solvents,
such as CH CN (73.2%), EtOH (74.6%), MeOH (68.2%)
3
and acetone (63.1%). Aprotic solvents oꢁered higher epoxy
cyclohexane selectivity (CH CN (98.2%), acetone (98.8%))
Based on the standard reaction conditions for the selective
oxidation of cyclohexene, the reaction scale was increased
by 100 times (Fig. S9). To eliminate the exothermic eꢁect
of hydrogen peroxide decomposition, hydrogen peroxide
was dripped at a rate of 3 mL/min by a peristaltic pump
in the experiment of 100 times expansion. Since hydrogen
peroxide was added slowly to the reaction solution, the
temperature of the reaction system was stably controlled at
308 K, and the water content was reduced in the initial stage
of the reaction, 89.0% cyclohexene conversion and 94.3%
epoxy cyclohexane selectivity were obtained at 360 min. The
results showed that the catalytic system performed well after
enlarging the reaction scale, and the selectivity of epoxy
cyclohexane was improved signiꢀcantly by controlling the
drop rate of hydrogen peroxide.
3
as compared to protic solvents (EtOH (85.4%), MeOH
(
59.2%)). Therefore, CH CN is the most suitable solvent
3
for cyclohexene epoxidation, which is both water-miscible
and aprotic. The eꢁect of the catalyst amount as illustrated
in Fig. S6. The cyclohexene conversion increased from 28.6
to 73.2%, and the epoxy cyclohexane selectivity increased
from 88.5 to 98.2% with increasing the amount of PS/PEI-
C -PW 125 to 500 mg at 180 min. This may be because
1
8
2
the resin’s pores can adsorb water, which reduced the polar-
ity of the solution and suppressed the hydrolysis of epoxy
cyclohexane. Figure S7 described the eꢁect of H O by
2
2
varying the H O /cyclohexene molar ratio from 0.5 to 2.0,
2
2
the cyclohexene conversion increased from 13.0 to 96.3%,
still, the epoxy cyclohexane selectivity decreased from 99.8
to 64.8% at 360 min, ascribable to the hydrolysis of epoxy
The activity of the PS/PEI-C -PW was still maintained
1
8
2
cyclohexane in the presence of excess H O. The optimal
at a high level after eight cycles (Table 3). As shown in Figs.
S3(G) and S4(D), the IR and XPS spectra of the recovered
catalyst were consistent with the fresh catalyst. These results
indicated that the catalyst could keep high recyclable stabil-
ity and has hopefully application foreground in industrial
production.
2
reaction temperature on the epoxidation of cyclohexene
was screened out by evaluating the catalytic performance
in temperature range from 298 to 308 K (Fig. S8). The
cyclohexene conversion increased from 45.0 to 73.2%, and
the epoxy cyclohexane selectivity was maintained above
1
3

J. Liu et al.
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1
8
2
cyclohexene
Catalyst
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.
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Conversion (%)
Selectivity (%)
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.
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WC, Yu R, Gu L, Dong JC, Luo J, Chen C, Peng Q, Draxl C,
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Fresh
73.2
74.2
76.5
72.5
76.0
72.1
73.4
69.7
71.3
98.2
91.3
94.5
96.5
96.1
98.7
97.6
98.4
96.4
Cycle1
Cycle2
Cycle3
Cycle4
Cycle5
Cycle6
Cycle7
Cycle8
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A series of modiꢀed chloromethyl polystyrene resin loaded
with various phosphotungstic heteropoly acid catalysts can
be synthesized by a simple method, and the PS/PEI-C -PW
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18
2
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exhibits excellent catalytic performance in cyclohexene
epoxidation using CH CN as solvent and 30 wt% H O as
3
2
2
2
2
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oxidant. 73.2% cyclohexene conversion and 98.2% epoxide
selectivity were obtained under mild conditions in 180 min.
8
9.0% cyclohexene conversion and 94.3% epoxide selectiv-
2
2
2
4. Zhao W, Ma B, Hua H, Zhang Y, Ding Y (2008) Catal Commun
9:2455–2459
ity can also be achieved by controlling the dropping rate
of H O after enlarging the reaction scale 100 times (in
5. Xia QH, Ge HQ, Ye CP, Liu ZM, Su KX (2005) Chem Rev
2
2
1
05:1603–1662
3
60 min). This catalyst has a good recovery, stability per-
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formance and a millimeter-sized particle size, which is quite
close to the industrial design requirement.
6
4:7267–7270
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2
8. Peng C, Lu XH, Ma XT, Shen Y, Wei CC, He J, Zhou D, Xia QH
Acknowledgements We gratefully acknowledge the grants from
the National Natural Science Foundation of China (Nos. 91634104
and 21776122) and National Key R&D Program of China
(
2016) J Mol Catal A 423:393–399
2
3
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No.2018ZX07208010).
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BC (1997) J Mol Catal A 117:185–198
Compliance with Ethical Standards
2. Shen Y, Lu XH, Wei CC, Ma XT (2017) Peng C, He J, Zhou D
and Xia QH. Mol Catal 433:185–192
Conflict of interest There are no conꢂicts to declare.
3. Kovalchuk T, Sfihi H, Zaitsev V, Fraissard J (2007) J Catal
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